Contact probes are used in the electrical circuit test industry. They provide electrical contact between a device under test (DUT) and a diagnostic or testing apparatus such as a test circuit board. These contact probes are normally housed in non-conductive test sockets, which acts an interface between the test circuit board and the DUT.
A conventional spring loaded contact probe generally includes an outer receptacle, a movable plunger, a barrel containing the plunger and a spring for biasing the reciprocating travel of the plunger in the barrel. The barrel is normally mounted within the receptacle with the plunger extending outwardly from the receptacle. All parts of the conventional spring loaded contact probe are preferably made of electrically conductive materials, so that an electrical circuit is maintained between the DUT and testing apparatus.
The barrel is normally crimped about a reduced area of the plunger, allowing the plunger to reciprocate relative to the barrel. Crimping requires an extra step of manufacture and is difficult in an item so small. Large surface area of contact is also highly desirable as electrical resistance is reduced with increase in surface area of contact. One of the objects of the invention is to provide an increased surface area of contact between plungers so that electrical resistance is greatly reduced.
The manufacturing of a barrel of a conventional spring loaded contact probe involves the extrusion of a metallic material into a hollow cylinder. This extrusion process results in rough surfaces on the hollow cylinder. These rough surface areas will result in reduction of surface area of contact and inconsistency of contact between the probes and the interior of the barrel when the probes are sliding within the barrel.
Components of the probe are normally plated with galvanic nickel, chemical nickel, hard gold, rhodium or silver to protect parts of the probe against corrosion and abrasion, as well as to reduce contact resistance due to rough surface area. Crevices on the probe and interior of the barrel are hard to reach places which plating thereon may not be possible. Furthermore, plating on the rough surfaces, which are a result of the extrusion process, does not eliminate problem of the surface being rough and uneven. Again, the problem of reduction of surface area of contact and inconsistency of contact between the probes and the interior of the barrel still exist.
In FIG. 1, an illustration is shown of a conventional spring loaded contact probe 160 found in the prior art. The contact probe 160 comprises of an electrically conducting barrel 110, a plunger 150 for contact with a DUT, a plunger 120 for contact with a circuit board, and a helical spring 130 coupled to both plungers 150, 120. The plunger 150 for contact with a DUT has a contact drill point 140 for maintaining contact with the DUT. The plunger 120 for contact with the circuit board has a contact drill point 190 for maintaining contact with the circuit board.
The plungers 150, 120 and the helical spring 130 are housed within the barrel 110. All parts of the contact probe 160 are axially aligned on a single axis. The plungers 150, 120 are slidably disposed within the barrel 110, which is crimped at both ends to limit the outwardly directed axial displacement of the plungers 150, 120 from the barrel 110. The barrel is necessary for electrical conductivity between both contacts. The surface areas of contact between the plungers 150, 120 and the barrel 110 are constant.
Conduction of electrical signals from one end of the plunger to the other end is through a metal plating on the inside of the barrel. Electrical resistance is inversely proportional to the square of the area of contact. By increasing the surface area of contact, there is less resistance. However, with constant surface area of contact between the plunger and barrel of the probe of the prior art, electrical resistance remains constant and may even increase with reduction of surface area of contact due to wear and tear. This is highly undesirable, as the reliability of the device will be greatly affected. Having the barrel in the probe of the prior art and the spring as a resilient means pose problems as discussed above.